Electrical insulation for high-voltage applications should ideally be completely solid and free of internal gas-filled voids. In practice, however, such insulation usually contains some internal voids due, for example, to bubbles from internally evolved gas during cure, to cracking or delamination from mechanical stress, or to incomplete impregnation of resin-impregnated systems. When alternating voltage is applied to such insulation, a portion of the voltage appears across these internal voids. At sufficiently high applied voltage, the voltage appearing across the void will cause electrical discharges in the gas within the void. These discharges, known as "partial discharges" or sometimes as "corona discharges", typically recurring once or more every half cycle of the applied voltage, will gradually erode the insulation and eventually result in failure. This is the primary failure mechanism for high-voltage electrical insulation, particularly for insulation systems involving organic resins.
Suppressing corona discharges would make it possible to increase the electrical lifetime of the insulation or to raise the voltage stress at which the insulation could be used, thereby permitting reduction of the required insulation thickness.
U.S. Pat. Nos. 3,622,524, 3,728,306, and 3,769,226, all in the name of Markovitz, et al., disclose that an electrical insulation material characterized by a good dissipation factor, a high heat distortion temperature and good corona resistance can be prepared by curing an epoxy resin with from about 20 to about 120 parts by weight per 100 parts of epoxy resin of an organotin compound that has been pre-reacted with an organic acid or anhydride material. Markovitz, et al. disclose that useful organotin compounds include dialkyltin and diaryltin oxides, organostannoic acids, organotin esters and organotin halides, which provide for ready entry of the tin into the organic insulation material.
U.S. Pat. No. 3,577,346, in the name of McKeown, discloses an insulating dielectric polymer that contains a minor amount of an organometallic compound of a metal selected from silicon, germanium, tin, lead, phosphorous, arsenic, antimony, bismuth, iron, ruthenium, and nickel. For example, McKeown discloses that the inclusion of ferrocene improves the corona life of a cured epoxy resin.
U.S. Pat. No. 4,173,593, in the name of Smith, et al., discloses that varnishes for coating large electrical component insulation can be prepared by reacting epoxy resin and maleic anhydride in the presence of a catalyst and then adding other ingredients such as styrene, along with a room temperature stabilizer, a polycarboxylic anhydride, a peroxide or azo compound as a catalyst, and an amount of a selected metal acetylacetonate acting as a latent accelerator.
It would be of great advantage to provide novel electrical insulation materials, particularly insulation materials having improved corona resistance relative to the insulation materials presently known in the art.